Field of the Invention
The present invention relates to a ventilation system for providing patient-triggered ventilatory support to a spontaneously breathing patient during ongoing high frequency ventilation (HFV) of the patient.
Description of the Prior Art
HFV ventilators supply breathing gas to the airways of a patient via a patient circuit from a frequency of approximately 150 breaths per minute up to 900 breaths per minute or more, and with tidal volumes significantly less than required during spontaneous breathing, typically at or below anatomical dead-space volumes. This is in marked contrast to a conventional mechanical ventilator which typically supplies breathing gas to the patient circuit at a frequency and with a tidal volume close to the values during spontaneous breathing.
An HFV ventilator generally comprises an oscillator which is connectable in gas communication with one end of the gas tubing of a patient circuit. The circuit terminates in an opposite end, such as in an endotracheal tube, for connection to a patient's airways. The oscillator is then driven to vibrate a column of gas within the circuit to actively supply gas to and sometimes also to extract gas from the airways of the patient. Typically, the HFV ventilator comprises a gas supply for providing a constant, continuous so called ‘bias’ flow to the patient. This bias flow intersects the oscillatory pathway and serves to maintain (bias) a mean positive airway pressure about which the high frequency oscillations generated by the HFV ventilator occurs and also to wash exhaled gasses from the circuit. Gas leaves the circuit through an expiratory limb, which may be designed as a low pass filter. The bias supply of such systems is usually insufficient to supply sufficient gas to a patient if the patient should attempt a spontaneous breath.
One known patient ventilator system, which reduces this problem, is disclosed in U.S. Pat. No. 5,165,398. The system comprises an HFV ventilator and a conventional mechanical ventilator connected to a patient breathing circuit and cooperable to provide, in one mode of operation, a conventional low frequency, large tidal, volume time cycled mechanical ventilator supply having superimposed thereon high frequency oscillations from the HFV ventilator. In another mode of operation this system can act as an HFV ventilator with the conventional mechanical ventilator providing the continuous bias flow at a level to maintain a constant pressure. A mechanical pressure regulator is provided in the patient circuit proximal the patient end which operates to increase this continuous bias flow and maintain the pressure as a patient attempts to breathe spontaneously. A non-assisted spontaneous breathing support mode of operation is thereby provided.
EP 1106197 discloses an HFV ventilation system capable of providing assisted support of a spontaneous breathing effort detected during high frequency oscillation ventilation. This is achieved by monitoring changes in one or both of the gas pressure and gas flow during the operation of an HFV ventilator, which changes are unrelated to the high frequency oscillations produced by that ventilator. Thereby, a spontaneous breathing effort can be detected and a gas supply, preferably a conventional mechanical ventilator, can be operated to supply breathing gas at a level to assist the detected spontaneous breathing effort. When a spontaneous breathing effort by a patient is detected the high frequency oscillations are reduced or removed from the gas in the patient circuit, which switches the operating mode of the ventilation system from an operation mode of an HFV ventilator to an operation mode of a conventional mechanical ventilator.
The proposed combination of HFV and pneumatically (flow or pressure) triggered ventilation support may, in certain circumstances and in certain types of ventilation systems, have the disadvantage of being imprecise due to the difficulty of accurately identifying and measuring the changes in flow or pressure caused by the patient's effort to breathe within the high frequency pressure oscillations caused by the HFV ventilator. Furthermore, it requires time trend analysis of the monitored pressure or flow in order to minimize the risk of mistaking gas leakage or hyperinflation for spontaneous breathing attempts by the patient.